In recent years, laser machining has become widely used as a machining method in the material processing field. For example, laser welding and laser cutting have been recognized as providing higher machining quality than the other methods. Under the circumstances, the market is requesting higher quality and faster machining. Specifically, the market is asking for laser oscillators and laser amplifiers that generate and emit single-mode laser with what is called high beam quality (i.e. high power, high efficiency, and high optical condensability).
To achieve high power, a conventional laser oscillator has a multi-core (multiple waveguides) structure in which fiber containing a laser medium and fiber for transmitting pumping light are positioned close to each other, and has a material with a given refractive index filling the space between the fiber (refer to patent literatures 1 and 2, for instance).
It is known that fiber wound at a certain curvature radius causes a bending loss, which allows the mode of transmitted light to be selected (refer to patent literature 3, for instance).
Meanwhile, the following method is known. That is, single-core (single waveguide) fiber is unified while the wrap-around curvature radius is being changed, and is irradiated with pumping light from the outside (patent literature 4, for instance).
FIG. 7A is a block diagram of the above-described conventional fiber laser apparatus. FIG. 7B is a sectional view taken along the line 7B-7B in FIG. 7A. In FIGS. 7A and 7B, pumping light transmission fiber 101 for transmitting pumping light and laser amplification fiber 102 containing a laser medium are placed close to each other. Bonding fiber 103 contains pumping light transmission fiber 101 and laser amplification fiber 102 and is filled with a material having a given refractive index.
One end of laser amplification fiber 102 has a final-stage mirror (not shown) for reflecting laser, and the other end has an output mirror (not shown) for extracting part of laser and reflecting the rest. The mirrors generate laser with multiplex feedback amplification.
A description is made of operation of the conventional laser oscillator configured as above. Pumping light propagating through pumping light transmission fiber 101 enters laser amplification fiber 102 through bonding fiber 103 to excite the laser medium. The excitation and multiplex feedback amplification cause laser to be generated and to be emitted from the output mirror.
To generate higher power, conventional fiber laser apparatus uses bonding fiber 103 with a multi-core structure, where bonding fiber 103 is filled with a resin filler. This resin imposes limitations on high-power light entering laser amplification fiber 102 from pumping light transmission fiber 101. Accordingly, to increase such high-power light, the fiber length needs to be increased, which prevents small-size, high-power fiber laser apparatus to be provided.